Fuel cells incorporating silica fibers

ABSTRACT

Embodiments of the invention include fuel cells incorporating sheets and/or powders of silica fibers and methods for producing such devices. The silica fibers may be formed via electrospinning of a sol gel produced with a silicon alkoxide reagent, such as tetraethyl ortho silicate, alcohol solvent, and an acid catalyst.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/578,915, filed Sep. 23, 2019, which claims the benefit of andpriority to U.S. Provisional Patent Application No. 62/735,419, filedSep. 24, 2018, the entire disclosure of each of which is herebyincorporated herein by reference.

TECHNICAL FIELD

In various embodiments, the present invention relates to fuel cellsincorporating silica fibers.

BACKGROUND

Fuel cell technology is of increasing interest due to its potential useas a high-efficiency, low-emission power source. Fuel cells areelectrochemical devices that convert chemical energy stored in fuelssuch as hydrogen into electrical power. Thin, membrane-based fuel cellsare receiving interest in applications such as portable power generationand fuel-cell vehicles. Various technologies for fuel cells have beenproposed, but the implementation of such technologies has been hamperedby poor reliability, low durability, or high cost. In addition, variousfuel-cell technologies are based on exotic and expensive materials,which are frequently toxic, thereby establishing another barrier toadoption.

In contrast, silicon dioxide, i.e., silica, is one of the most abundantmaterials on Earth, being the major component of most types of sand.Silica has several advantageous properties that have resulted in its usein many different industries and products. For example, the highelectrical resistance of silica has enabled its use as ahigh-performance insulator in microelectronic devices, e.g., as thegate-dielectric material in field-effect transistors. Silica is alsoutilized in the production of glass usable in many differentapplications. Optical fibers, for example, are fabricated utilizingsilica and have enabled the formation and growth of worldwide opticaltelecommunications networks. Silica has also been utilized at themicroscopic scale, as silica particles have been utilized as abrasiveagents, as desiccants, and to form molds for investment casting ofmetallic materials. However, silica has yet to be utilized as a primarybackbone material in fuel cells or similar devices. Utilization ofsilica may enable the fabrication of and use of fuel cells that are morefriendly to the environment and that may be utilized to satisfy therequirements of next-generation power generation.

SUMMARY

In accordance with various embodiments of the present invention, mats ofsilica fibers, portions thereof, and/or powders fabricated therefrom,are utilized as or in the structural matrix for one or more componentsof a fuel cell. Various components of the fuel cell may incorporateother materials applied to and/or within the silica fibers in order toenable the desired conductivity or other properties of the fuel cell.The silica fibers themselves may be produced from a gelatinous materialthat is electrospun to form a fiber mat. The mat itself (or a portionthereof) may be utilized within the fuel cell, with or withoutadditional processing (e.g., pressing and/or incorporation of anadditive material). In various embodiments, the mat is fragmented into apowder or dust, which may include, consist essentially of, or consist offibrous fragments. The powder, which may already incorporate one or moreadditive materials introduced before, during, or after the fiberelectrospinning process, may be utilized in one or more regions of thefuel cells. In various embodiments, the powder is mixed with one or moreadditives for use in one or more fuel-cell regions. In otherembodiments, the powder is pressed into a planar sheet and utilizedwithin the fuel cell, with or without the incorporation of one or moreadditives.

In various embodiments, the silica fibers may be prepared byelectrospinning a sol-gel, which may be prepared with a silicon alkoxidereagent, such as tetraethyl ortho silicate (TEOS), alcohol solvent, andan acid catalyst. In various embodiments, the sol-gel is produced viaripening of sol under controlled environmental conditions, and/or theproperties of the sol or sol-gel during the ripening process aremonitored, in order to identify various processing windows during whichthe electrospinning of the sol-gel may be successfully performed. Asknown in the art, a “sol” is a colloidal solution that gradually evolvestowards the formation of a “gel,” i.e., a diphasic system containingboth a liquid phase and solid phase. Herein, the term “sol-gel” is usedto refer to the gel produced from the sol-gel process that may beelectrospun into fibers or a fibrous mat.

In various embodiments, the controlled environment for ripening the solmay involve controlled conditions in terms of humidity, temperature, andoptionally barometric pressure. For example, the humidity may becontrolled within the range of about 30% to about 90%, and thetemperature may be controlled within the range of from about 50° F. toabout 90° F. By controlling the environmental conditions duringripening, the gel may be electrospun during the time when spinning isoptimal, which can occur in a very small window of only several minutesif the ripening process is accelerated by direct heat. When ripening thesol at a constant humidity in the range of about 50% to 80% and atemperature of about 60 to 80° F., the sol will ripen (gelatinize) in afew days, and the window for successful electrospinning may be expandedto at least several hours, and in some embodiments several days. The solmay therefore be ripened in an enclosure which may include one or moreenvironmental monitors, such as a temperature reading device and/or ahumidity reading device. Further, gases produced or released by the solduring the ripening process and/or relative weight of the sol may bemonitored to determine a suitable or optimal time for electrospinning.

Once the sol is adequately ripened into a sol-gel, it is electrospun toform a mat of entangled silica fibers. Once electrospun, the silicafibers may have a variable diameter, such as in the range of from about50 nm to 5 μm. In some embodiments, the fibers are predominately in therange of about 100 nm to about 2 μm, or predominately in the range ofabout 200 to about 1000 nm. For fabrication of various regions of thefuel cell, different materials may be applied to the silica fibersduring and/or after the electrospinning process in order to imbue theresulting fibers, fiber mats, or powder with different properties. Forexample, carbon (e.g., in the form of carbon black, graphite, graphene,or carbon nanoparticles) and/or one or more other electricallyconductive materials may be applied to the electrospun orelectrospinning silica fibers in order to increase the electricalconductivity of the silica fiber mat; such fibers may be utilized inand/or fragmented into a powder for use in an electrode of a fuel cell.In addition, a reaction catalyst (e.g., platinum metal, one or moreplatinum-group metals, ruthenium, nickel, palladium, cerium oxide, or analloy of platinum with one or more other metals such as cobalt, nickel,iron, vanadium, manganese, and/or chromium, etc.) may be applied to thesilica fibers to enable the necessary reactivity of the fuel cell; suchfibers may be used directly in the fuel cell or pressed into sheets orfragmented into a powder for use in the fuel cell. In variousembodiments, additives or functional materials such as the electricallyconductive material and/or the reaction catalyst may be incorporatedinto (e.g., mixed with) powder formed via fragmentation of theelectrospun fibers.

In various embodiments, one or more additives or functional materialsare added into the ripening sol (for example, in aqueous and/orsolid/crystalline form) prior to electrospinning. For example, additiveprecursors such as nickel chloride, palladium chloride, and/orchloroplatinic acid may be added into the sol prior to electrospinning.During spinning, the additive(s) from the precursors (e.g., one or moremetals) are incorporated into and/or on the electrospun fibers.

Fuel cells in accordance with embodiments of the invention mayincorporate electrolyte layers that enable proton conduction through thefuel cell for generation of electricity. In various embodiments of theinvention, the electrolyte layer may include, consist essentially of, orconsist of a mat of silica fibers incorporating an additive thatfacilitates proton conduction through the electrolyte. For example, suchelectrolyte additives may include potassium hydroxide and/or phosphoricacid. In various embodiments, the electrolyte layer may include, consistessentially of, or consist of one or more polymeric layers that aretypically porous to enable proton transfer. For example, pores in apolymeric layer may range in size from approximately 5 nm toapproximately 100 nm. Such polymeric layers may include, consistessentially of, or consist of, for example, one or more polymericmaterials such as polyethylene, polypropylene, polytetrafluoroethylene,polyvinyl chloride, and/or polymer blends including one or more of thesewith or without one or more other polymeric materials. The polymericlayer may include powder fabricated from silica fibers incorporatedtherein or thereon, and/or the polymeric layer may include one or moresheets of silica fibers incorporated thereon; as detailed herein, thesilica fibers may have one or more functional materials incorporatedtherein. In an exemplary embodiment, the electrolyte layer includes,consists essentially of, or consists of a sheet or mat of silica fibersdisposed (e.g., pressed) between two polymer layers. The electrolyteadditive may be incorporated into the middle sheet or mat of silicafibers.

As utilized herein, a “sheet” of silica fibers refers to an electrospunmat of silica fibers, with or without additional pressing or processing,a pressed mat of silica fibers, or to a pressed layer of powder (e.g.,fibrous fragments) formed via fragmentation of electrospun silica fibermats. For example, one or more sheets of silica fibers may be utilizedas or as a portion of an electrolyte layer, a catalyst layer, and/or adiffusion layer in fuel cells in accordance with embodiments of thepresent invention. In various embodiments, the incorporation of one ormore sheets of silica fibers provides the fuel cell with protection fromthermal decomposition and/or deformation, enhances mechanical integrityof the structure inside the fuel cell, and/or improves charge mobilitywithin the fuel cell.

As utilized herein, in a region of a fuel cell “incorporating” anadditive material such as one of the various materials listed above andherein (e.g., a conductive additive such as carbon and/or a catalystadditive) in or on the region (e.g., in or on the silica fibers and/orsilica fiber powder), the additive material may be bonded to orotherwise adhered to in a substantially solid form to the fibers orpowder particles, present within the crystalline structure of the fibersor powder particles themselves, and/or present within a mat or sheet(e.g., within pores or spaces between fibers) or within a collection ofpowder particles as a solid or in liquid form (e.g., with a liquid orsolid binder or carrier such an organic liquid such as propylenecarbonate and/or other organic polymers mentioned herein).

Various embodiments of the invention have advantageous properties whenoperating as fuel cells. For example, the layers of silica fibers haveporosity that enables the efficient transport of fuel, oxygen, andreaction by-products through portions of the fuel cell while stillproviding sufficient conductivity for high performance. The porosity mayalso accommodate thermal cycles of the fuel cell while limitingdeformation of the device. In addition, the silica fiber networkutilized in the various layers of fuel cells in accordance withembodiments of the invention has a large surface area (e.g., rangingfrom approximately 50 m²/gram to approximately 100 m²/gram), therebyenabling a large reaction volume during device operation. The silicafiber networks are also advantageously thermally insulating and thuswill thermally shield various layers of the fuel cell from extremeenvironmental conditions, thereby increasing the lifetime of the fuelcell. Moreover, fuel cells in accordance with embodiments of the presentinvention have a high moisture retention capability, to thereforepreserve therewithin additives such as liquid electrolyte additivesduring operation. Fuel cells in accordance with embodiments of theinvention may be advantageously utilized over a wide range of operatingtemperatures, as the silica fibers are non-reactive and have a highchemical stability. Fuel cells in accordance with embodiments of theinvention may also be utilized at low levels of catalyst (e.g., metalcatalyst) loading, making them extremely cost effective. Fuel cells inaccordance with embodiments of the invention are also scalable andeasily processed into many shapes and sizes.

In various embodiments of the invention, one or more regions or layersof the fuel cell include, consist essentially of, or consist of silicafiber powder (with or without an additive). For example, in variousembodiments, once a silica fiber mat is successfully electrospun, it maybe processed into a powder or dust. For example, the electrospun mat maybe “fragmented,” i.e., fractured, cut, ground, milled (e.g., in a ballmill or other milling device), pulverized, or otherwise divided intosmall fragments that maintain a fibrous structure. As used herein, theterm “fibrous fragments” (or “fibrous-mat fragments,” or simply“fragments”) refers to small particles, parts, or flakes of a fibrousmat having an average dimension larger (e.g., 5×, 10×, or even 100×)than the width of at least some of the fibers of the mat. In variousembodiments, the average size of a fibrous fragment is in the range ofapproximately 20 μm to approximately 200 μm. Fibrous fragments may thusresemble microscopic-scale versions of the electrospun mat itself e.g.,intertwined collections of silica fibers, and thus typically are porousand have low densities. Thus, fibrous fragments may be contrasted withother types of micro-scale particles, such as the substantiallyspherical particles used in colloidal silica, which are each unitary,individual units or grains, rather than small collections of fibers.Various portions of a fibrous fragment (e.g., the edges) may have sharpand/or broken edges resulting from the fracturing process utilized toform the fragments from the electrospun mat. As utilized herein, theterms “silica fiber powder,” “silica powder,” “silica dust,” and “fiberdust” include collections of particles generated via the fragmentationof electrospun fiber mats and/or fibers, and may include fibrousfragments and/or other powder particles resulting from suchfragmentation.

Embodiments of the present invention may employ silica fibers, fragmentsthereof, and/or mixtures incorporating such fibers or fragments, and/ormethods for fabricating such fibers or fragments detailed in U.S. patentapplication Ser. No. 15/934,599, filed on Mar. 23, 2018 (issued as U.S.Pat. No. 10,111,783), U.S. patent application Ser. No. 16/131,531, filedon Sep. 14, 2018, U.S. patent application Ser. No. 16/353,181, filed onMar. 14, 2019, and U.S. patent application Ser. No. 16/367,313, filed onMar. 28, 2019, the entire disclosure of each of which is incorporated byreference herein.

In an aspect, embodiments of the invention feature a method offabricating a fuel cell. A first sheet of silica fibers is formed atleast in part by electrospinning a sol-gel. An electrolyte layer isformed at least in part by, after and/or during the electrospinning,incorporating a first functional material onto the first sheet of silicafibers. The electrolyte layer may be configured to conduct protonstherethrough. The electrolyte layer may be configured to conduct protonstherethrough while preventing the conduction of electrons therethrough.A first catalyst layer is disposed on an anode side of the electrolytelayer. The first catalyst layer may be configured to receive ahydrogen-containing fuel, oxidize hydrogen from the fuel, and supplyprotons to the electrolyte layer. A second catalyst layer is disposed ona cathode side of the electrolyte layer. The anode side and the cathodeside may be on opposite sides of the electrolyte layer. The secondcatalyst layer may be configured to receive protons from the electrolytelayer, receive oxygen, and reduce the oxygen to form water to therebyenable the production of electrical current by the fuel cell.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The first catalyst layer and/or thesecond catalyst layer may include, consist essentially of, or consist ofa support layer incorporating a catalyst material. The catalyst materialmay include, consist essentially of, or consist of platinum, one or moreplatinum-group metals, nickel, ruthenium, palladium, cerium oxide,and/or an alloy of platinum with one or more other metals. The supportlayer may include, consist essentially of, or consist of a sheet ofsilica fibers. The support layer may include, consist essentially of, orconsist of carbon or a carbon-containing material. The first catalystlayer may include, consist essentially of, or consist of a second sheetof silica fibers. The first catalyst layer may be provided at least inpart by electrospinning a second sol-gel. One or more catalystmaterials, or precursors thereof, may be incorporated into the secondsol-gel prior to electrospinning thereof. The second sheet of silicafibers may include the one or more catalyst materials incorporatedtherein and/or thereon. The second catalyst layer may include, consistessentially of, or consist of a third sheet of silica fibers. The secondcatalyst layer may be provided at least in part by electrospinning athird sol-gel. One or more catalyst materials, or precursors thereof,may be incorporated into the third sol-gel prior to electrospinningthereof. The third sheet of silica fibers may include the one or morecatalyst materials incorporated therein and/or thereon. Two or more ofthe sol-gel, the second sol-gel, and the third sol-gel may havesubstantially the same composition. Two or more of the sol-gel, thesecond sol-gel, and the third sol-gel may have different compositions.

A first diffusion layer may be disposed on the first catalyst layeropposite the electrolyte layer. The first diffusion layer may beconfigured to transport the hydrogen-containing fuel to the firstcatalyst layer. A second diffusion layer may be disposed on the secondcatalyst layer opposite the electrolyte layer. The second diffusionlayer may be configured to transport oxygen to the second catalystlayer. The first diffusion layer may include, consist essentially of, orconsist of a second sheet of silica fibers. The first diffusion layermay be provided at least in part by electrospinning a second sol-gel.The second diffusion layer may include, consist essentially of, orconsist of a third sheet of silica fibers. The second diffusion layermay be provided at least in part by electrospinning a third sol-gel. Thefirst functional material may include, consist essentially of, orconsist of a fluoropolymer, potassium hydroxide, and/or phosphoric acid.

The sol-gel may be prepared with tetraethylorthosilicate (TEOS). Priorto electrospinning the sol-gel, the sol-gel may be produced from aninitial sol containing 75% to 90% TEOS, 8% to 25% ethanol, an acidcatalyst, and the balance water. The initial sol may comprise, consistessentially of, or consist of 70% to 90% TEOS by weight, 8% to 25%ethanol by weight, an acid catalyst, and water. The initial sol maycontain 70% to 90% TEOS by weight, 8% to 25% ethanol by weight, an acidcatalyst, and the balance water. The initial sol may comprise, consistessentially of, or consist of 70% to 90% TEOS by weight, 8% to 25%ethanol by weight, an acid catalyst, and water. The initial sol maycontain 70% to 90% TEOS by weight, 8% to 25% ethanol by weight, an acidcatalyst, and the balance water. The initial sol may comprise, consistessentially of, or consist of 70% to 90% TEOS by weight, 8% to 25%ethanol by weight, an acid catalyst, and water. The initial sol maycomprise, consist essentially of, or consist of 70% to 90% TEOS byweight, 8% to 25% ethanol by weight, 1% to 10% water by weight, and theacid catalyst. The initial sol may comprise, consist essentially of, orconsist of 75% to 85% by weight TEOS, 12% to 20% by weight ethanol, andabout 2% to 5% by weight water. The initial sol may comprise, consistessentially of, or consist of about 80% by weight TEOS, about 17% byweight ethanol, and about 3% by weight water. The acid catalyst maycomprise, consist essentially of, or consist of HCl. The initial sol maycontain less than about 0.1% of the acid catalyst by weight. The initialsol may contain from 0.02% to 0.08% of the acid catalyst by weight. Theinitial sol may contain one or more reagents that alter one or moreproperties of the initial sol, the sol-gel, and/or the silica fibers.

Producing the sol-gel may include transitioning (or ripening) theinitial sol for at least 2 days under conditions where humidity iswithin the range of about 40% to about 80%, and the temperature iswithin the range of 50° F. to 90° F. The initial sol may be allowed totransition for at least 3 days, at least 4 days, at least 5 days, atleast 6 days, or at least 7 days. The initial sol may be allowed totransition for 2 days to 10 days, and for 2 days to 7 days in someembodiments. The sol-gel may be electrospun when the weight is at from10% to 60% of the starting weight of the initial sol or sol-gel beforeripening (transitioning). The sol-gel may be electrospun when the weightis at from 10% to 40% of the starting weight of the initial sol orsol-gel before ripening (transitioning). The sol-gel may be electrospunwhen the weight is at from 20% to 40% of the starting weight of theinitial sol or sol-gel before ripening (transitioning). The sol-gel maybe electrospun when the production of ethylene vapor is 10% to 20%relative to the peak production of ethylene vapors during ripening(transitioning) of the initial sol or sol-gel before ripening. Thesol-gel may be electrospun when the production of ethylene vaportherefrom is 10% to 40% relative to the initial sol or sol-gel beforeripening (transitioning).

Forming the electrolyte layer may include disposing the first sheet ofsilica fibers adjacent to, proximate, and/or in contact with one or morenon-silica membranes. Forming the electrolyte layer may includedisposing the first sheet of silica fibers between two non-silicamembranes. The first sheet of silica fibers may be in direct contactwith the two non-silica membranes. The two non-silica membranes mayinclude, consist essentially of, or consist of the same material ordifferent materials. The one or more non-silica membranes may include,consist essentially of, or consist of a polymeric material. Embodimentsof the invention include fuel cells fabricated in accordance with any ofthe methods described above.

In another aspect, embodiments of the invention feature a method offabricating a fuel cell. An electrolyte layer is provided. Theelectrolyte layer may be configured to conduct protons therethrough. Theelectrolyte layer may be configured to conduct protons therethroughwhile preventing the conduction of electrons therethrough. A firstcatalyst layer is provided. The first catalyst layer may be configuredto receive a hydrogen-containing fuel, oxidize hydrogen from the fuel,and supply protons to the electrolyte layer. A second catalyst layer isprovided. The second catalyst layer may be configured to receive protonsfrom the electrolyte layer, receive oxygen, and reduce the oxygen toform water to thereby enable the production of electrical current by thefuel cell. The first catalyst layer may be disposed on an anode side ofthe electrolyte layer. The second catalyst layer may be disposed on acathode side of the electrolyte layer. The anode side and the cathodeside may be opposite sides of the electrolyte layer. The first catalystlayer may include, consist essentially of, or consist of a first sheetof silica fibers. The first catalyst layer may be provided at least inpart by electrospinning a first sol-gel. The second catalyst layer mayinclude, consist essentially of, or consist of a second sheet of silicafibers. The second catalyst layer may be provided at least in part byelectrospinning a second sol-gel.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The first catalyst layer and/or thesecond catalyst layer may incorporate a catalyst material. The catalystmaterial may include, consist essentially of, or consist of platinum,one or more platinum-group metals, nickel, ruthenium, palladium, ceriumoxide, and/or an alloy of platinum with one or more other metals.Providing the first catalyst layer may include, after and/or during theelectrospinning, incorporating a catalyst material onto the first sheetof silica fibers. Providing the second catalyst layer may include, afterand/or during the electrospinning, incorporating a catalyst materialonto the second sheet of silica fibers. One or more catalyst materials,or precursors thereof (e.g., solutions, salts, and/or compoundscontaining the catalyst materials), may be incorporated into the firstsol-gel prior to electrospinning thereof. The first sheet of silicafibers may include the one or more catalyst materials incorporatedtherein and/or thereon. One or more catalyst materials, or precursorsthereof (e.g., solutions, salts, and/or compounds containing thecatalyst materials), may be incorporated into the second sol-gel priorto electrospinning thereof. The second sheet of silica fibers mayinclude the one or more catalyst materials incorporated therein and/orthereon. The electrolyte layer may include, consist essentially of, orconsist of a non-silica membrane, a polymer material, a fluoropolymer,potassium hydroxide, and/or phosphoric acid.

A first diffusion layer may be disposed on the first catalyst layeropposite the electrolyte layer. The first diffusion layer may beconfigured to transport the hydrogen-containing fuel to the firstcatalyst layer. A second diffusion layer may be disposed on the secondcatalyst layer opposite the electrolyte layer. The second diffusionlayer may be configured to transport oxygen to the second catalystlayer. The first diffusion layer may include, consist essentially of, orconsist of a third sheet of silica fibers. The first diffusion layer maybe provided at least in part by electrospinning a third sol-gel. Thesecond diffusion layer may include, consist essentially of, or consistof a fourth sheet of silica fibers. The second diffusion layer may beprovided at least in part by electrospinning a fourth sol-gel. Two ormore of the sol-gel, the second sol-gel, the third sol-gel, and thefourth sol-gel may have substantially the same composition. Two or moreof the sol-gel, the second sol-gel, the third sol-gel, and the fourthsol-gel may have different compositions.

For one or more of the sol-gels mentioned above, the sol-gel may beprepared with tetraethylorthosilicate (TEOS). Prior to electrospinningthe sol-gel, the sol-gel may be produced from an initial sol containing75% to 90% TEOS, 8% to 25% ethanol, an acid catalyst, and the balancewater. The initial sol may comprise, consist essentially of, or consistof 70% to 90% TEOS by weight, 8% to 25% ethanol by weight, an acidcatalyst, and water. The initial sol may contain 70% to 90% TEOS byweight, 8% to 25% ethanol by weight, an acid catalyst, and the balancewater. The initial sol may comprise, consist essentially of, or consistof 70% to 90% TEOS by weight, 8% to 25% ethanol by weight, an acidcatalyst, and water. The initial sol may contain 70% to 90% TEOS byweight, 8% to 25% ethanol by weight, an acid catalyst, and the balancewater. The initial sol may comprise, consist essentially of, or consistof 70% to 90% TEOS by weight, 8% to 25% ethanol by weight, an acidcatalyst, and water. The initial sol may comprise, consist essentiallyof, or consist of 70% to 90% TEOS by weight, 8% to 25% ethanol byweight, 1% to 10% water by weight, and the acid catalyst. The initialsol may comprise, consist essentially of, or consist of 75% to 85% byweight TEOS, 12% to 20% by weight ethanol, and about 2% to 5% by weightwater. The initial sol may comprise, consist essentially of, or consistof about 80% by weight TEOS, about 17% by weight ethanol, and about 3%by weight water. The acid catalyst may comprise, consist essentially of,or consist of HCl. The initial sol may contain less than about 0.1% ofthe acid catalyst by weight. The initial sol may contain from 0.02% to0.08% of the acid catalyst by weight. The initial sol may contain one ormore reagents that alter one or more properties of the initial sol, thesol-gel, and/or the silica fibers.

Producing the sol-gel may include transitioning (or ripening) theinitial sol for at least 2 days under conditions where humidity iswithin the range of about 40% to about 80%, and the temperature iswithin the range of 50° F. to 90° F. The initial sol may be allowed totransition for at least 3 days, at least 4 days, at least 5 days, atleast 6 days, or at least 7 days. The initial sol may be allowed totransition for 2 days to 10 days, and for 2 days to 7 days in someembodiments. The sol-gel may be electrospun when the weight is at from10% to 60% of the starting weight of the initial sol or sol-gel beforeripening (transitioning). The sol-gel may be electrospun when the weightis at from 10% to 40% of the starting weight of the initial sol orsol-gel before ripening (transitioning). The sol-gel may be electrospunwhen the weight is at from 20% to 40% of the starting weight of theinitial sol or sol-gel before ripening (transitioning). The sol-gel maybe electrospun when the production of ethylene vapor is 10% to 20%relative to the peak production of ethylene vapors during ripening(transitioning) of the initial sol or sol-gel before ripening. Thesol-gel may be electrospun when the production of ethylene vaportherefrom is 10% to 40% relative to the initial sol or sol-gel beforeripening (transitioning). Embodiments of the invention include fuelcells fabricated in accordance with any of the methods described above.

In yet another aspect, embodiments of the invention feature a fuel cellincluding, consisting essentially of, or consisting of an electrolytelayer, a first catalyst layer disposed on an anode side of theelectrolyte layer, and a second catalyst layer disposed on a cathodeside of the electrolyte layer. The electrolyte layer includes, consistsessentially of, or consists of a first sheet of silica fibers. Theelectrolyte layer may have a first functional material incorporatedwithin the first sheet of silica fibers. The first catalyst layer may beconfigured to receive a hydrogen-containing fuel, oxidize hydrogen fromthe fuel, and supply protons to the electrolyte layer. The secondcatalyst layer may be configured to receive protons from the electrolytelayer, receive oxygen, and reduce the oxygen to form water to therebyenable the production of electrical current by the fuel cell.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The first catalyst layer and/or thesecond catalyst layer may include, consist essentially of, or consist ofa support layer incorporating a catalyst material. The catalyst materialmay include, consist essentially of, or consist of platinum, one or moreplatinum-group metals, nickel, ruthenium, palladium, cerium oxide,and/or an alloy of platinum with one or more other metals. The supportlayer may include, consist essentially of, or consist of a second sheetof silica fibers and/or carbon or a carbon-containing material. Thefirst catalyst layer may include, consist essentially of, or consist ofa second sheet of silica fibers incorporating a first catalyst material.The second catalyst layer may include, consist essentially of, orconsist of a third sheet of silica fibers incorporating a secondcatalyst material. The first catalyst material and/or the secondcatalyst material may include, consist essentially of, or consist ofplatinum, one or more platinum-group metals, nickel, ruthenium,palladium, cerium oxide, and/or an alloy of platinum with one or moreother metals. A first diffusion layer may be disposed on the firstcatalyst layer opposite the electrolyte layer. The first diffusion layermay be configured to transport the hydrogen-containing fuel to the firstcatalyst layer. A second diffusion layer may be disposed on the secondcatalyst layer opposite the electrolyte layer. The second diffusionlayer may be configured to transport oxygen to the second catalystlayer. The first diffusion layer may include, consist essentially of, orconsist of a second sheet of silica fibers. The second diffusion layermay include, consist essentially of, or consist of a third sheet ofsilica fibers.

The first functional material may include, consist essentially of, orconsist of a polymer material, a fluoropolymer, potassium hydroxide,and/or phosphoric acid. The electrolyte layer may include one or morenon-silica membranes adjacent to, proximate, and/or in contact with thefirst sheet of silica fibers. The electrolyte layer may include twonon-silica membranes sandwiching the first sheet of silica fiberstherebetween. The one or more non-silica membranes may include, consistessentially of, or consist of a polymeric material.

The first sheet of silica fibers may be formed at least in part byelectrospinning a sol-gel. The sol-gel may be prepared withtetraethylorthosilicate (TEOS). Prior to electrospinning the sol-gel,the sol-gel may be produced from an initial sol containing 75% to 90%TEOS, 8% to 25% ethanol, an acid catalyst, and the balance water. Theinitial sol may comprise, consist essentially of, or consist of 70% to90% TEOS by weight, 8% to 25% ethanol by weight, an acid catalyst, andwater. The initial sol may contain 70% to 90% TEOS by weight, 8% to 25%ethanol by weight, an acid catalyst, and the balance water. The initialsol may comprise, consist essentially of, or consist of 70% to 90% TEOSby weight, 8% to 25% ethanol by weight, an acid catalyst, and water. Theinitial sol may contain 70% to 90% TEOS by weight, 8% to 25% ethanol byweight, an acid catalyst, and the balance water. The initial sol maycomprise, consist essentially of, or consist of 70% to 90% TEOS byweight, 8% to 25% ethanol by weight, an acid catalyst, and water. Theinitial sol may comprise, consist essentially of, or consist of 70% to90% TEOS by weight, 8% to 25% ethanol by weight, 1% to 10% water byweight, and the acid catalyst. The initial sol may comprise, consistessentially of, or consist of 75% to 85% by weight TEOS, 12% to 20% byweight ethanol, and about 2% to 5% by weight water. The initial sol maycomprise, consist essentially of, or consist of about 80% by weightTEOS, about 17% by weight ethanol, and about 3% by weight water. Theacid catalyst may comprise, consist essentially of, or consist of HCl.The initial sol may contain less than about 0.1% of the acid catalyst byweight. The initial sol may contain from 0.02% to 0.08% of the acidcatalyst by weight. The initial sol may contain one or more reagentsthat alter one or more properties of the initial sol, the sol-gel,and/or the silica fibers.

Producing the sol-gel may include transitioning (or ripening) theinitial sol for at least 2 days under conditions where humidity iswithin the range of about 40% to about 80%, and the temperature iswithin the range of 50° F. to 90° F. The initial sol may be allowed totransition for at least 3 days, at least 4 days, at least 5 days, atleast 6 days, or at least 7 days. The initial sol may be allowed totransition for 2 days to 10 days, and for 2 days to 7 days in someembodiments. The sol-gel may be electrospun when the weight is at from10% to 60% of the starting weight of the initial sol or sol-gel beforeripening (transitioning). The sol-gel may be electrospun when the weightis at from 10% to 40% of the starting weight of the initial sol orsol-gel before ripening (transitioning). The sol-gel may be electrospunwhen the weight is at from 20% to 40% of the starting weight of theinitial sol or sol-gel before ripening (transitioning). The sol-gel maybe electrospun when the production of ethylene vapor is 10% to 20%relative to the peak production of ethylene vapors during ripening(transitioning) of the initial sol or sol-gel before ripening. Thesol-gel may be electrospun when the production of ethylene vaportherefrom is 10% to 40% relative to the initial sol or sol-gel beforeripening (transitioning).

In another aspect, embodiments of the invention feature a fuel cellincluding, consisting essentially of, or consisting of an electrolytelayer, a first catalyst layer disposed on an anode side of theelectrolyte layer, and a second catalyst layer disposed on a cathodeside of the electrolyte layer. The first catalyst layer may beconfigured to receive a hydrogen-containing fuel, oxidize hydrogen fromthe fuel, and supply protons to the electrolyte layer. The secondcatalyst layer may be configured to receive protons from the electrolytelayer, receive oxygen, and reduce the oxygen to form water to therebyenable the production of electrical current by the fuel cell. The firstcatalyst layer includes, consists essentially of, or consists of a firstsheet of silica fibers incorporating a first catalyst material, and/orthe second catalyst layer includes, consists essentially of, or consistsof a second sheet of silica fibers incorporating a second catalystmaterial.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The electrolyte layer may include,consist essentially of, or consist of a non-silica membrane, a polymermaterial, a fluoropolymer, potassium hydroxide, and/or phosphoric acid.The first catalyst material and/or the second catalyst material mayinclude, consist essentially of, or consist of platinum, one or moreplatinum-group metals, nickel, ruthenium, palladium, cerium oxide,and/or an alloy of platinum with one or more other metals. A firstdiffusion layer may be disposed on the first catalyst layer opposite theelectrolyte layer. The first diffusion layer may be configured totransport the hydrogen-containing fuel to the first catalyst layer. Asecond diffusion layer may be disposed on the second catalyst layeropposite the electrolyte layer. The second diffusion layer may beconfigured to transport oxygen to the second catalyst layer. The firstdiffusion layer may include, consist essentially of, or consist of athird sheet of silica fibers. The second diffusion layer may include,consist essentially of, or consist of a fourth sheet of silica fibers.

The first sheet of silica fibers and/or the second sheet of silicafibers may be formed at least in part by electrospinning a sol-gel. Thesol-gel may be prepared with tetraethyl orthosilicate (TEOS). Prior toelectrospinning the sol-gel, the sol-gel may be produced from an initialsol containing 75% to 90% TEOS, 8% to 25% ethanol, an acid catalyst, andthe balance water. The initial sol may comprise, consist essentially of,or consist of 70% to 90% TEOS by weight, 8% to 25% ethanol by weight, anacid catalyst, and water. The initial sol may contain 70% to 90% TEOS byweight, 8% to 25% ethanol by weight, an acid catalyst, and the balancewater. The initial sol may comprise, consist essentially of, or consistof 70% to 90% TEOS by weight, 8% to 25% ethanol by weight, an acidcatalyst, and water. The initial sol may contain 70% to 90% TEOS byweight, 8% to 25% ethanol by weight, an acid catalyst, and the balancewater. The initial sol may comprise, consist essentially of, or consistof 70% to 90% TEOS by weight, 8% to 25% ethanol by weight, an acidcatalyst, and water. The initial sol may comprise, consist essentiallyof, or consist of 70% to 90% TEOS by weight, 8% to 25% ethanol byweight, 1% to 10% water by weight, and the acid catalyst. The initialsol may comprise, consist essentially of, or consist of 75% to 85% byweight TEOS, 12% to 20% by weight ethanol, and about 2% to 5% by weightwater. The initial sol may comprise, consist essentially of, or consistof about 80% by weight TEOS, about 17% by weight ethanol, and about 3%by weight water. The acid catalyst may comprise, consist essentially of,or consist of HCl. The initial sol may contain less than about 0.1% ofthe acid catalyst by weight. The initial sol may contain from 0.02% to0.08% of the acid catalyst by weight. The initial sol may contain one ormore reagents that alter one or more properties of the initial sol, thesol-gel, and/or the silica fibers.

Producing the sol-gel may include transitioning (or ripening) theinitial sol for at least 2 days under conditions where humidity iswithin the range of about 40% to about 80%, and the temperature iswithin the range of 50° F. to 90° F. The initial sol may be allowed totransition for at least 3 days, at least 4 days, at least 5 days, atleast 6 days, or at least 7 days. The initial sol may be allowed totransition for 2 days to 10 days, and for 2 days to 7 days in someembodiments. The sol-gel may be electrospun when the weight is at from10% to 60% of the starting weight of the initial sol or sol-gel beforeripening (transitioning). The sol-gel may be electrospun when the weightis at from 10% to 40% of the starting weight of the initial sol orsol-gel before ripening (transitioning). The sol-gel may be electrospunwhen the weight is at from 20% to 40% of the starting weight of theinitial sol or sol-gel before ripening (transitioning). The sol-gel maybe electrospun when the production of ethylene vapor is 10% to 20%relative to the peak production of ethylene vapors during ripening(transitioning) of the initial sol or sol-gel before ripening. Thesol-gel may be electrospun when the production of ethylene vaportherefrom is 10% to 40% relative to the initial sol or sol-gel beforeripening (transitioning).

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “approximately,” “about,” and “substantially” mean ±10%, and insome embodiments, ±5%. The term “consists essentially of” meansexcluding other materials that contribute to function, unless otherwisedefined herein. Nonetheless, such other materials may be present,collectively or individually, in trace amounts. Unless otherwiseindicated, fuel cells, materials, mixtures, regions, and otherstructures described herein may incorporate unintentional impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A is a cross-sectional schematic of a fuel cell in accordance withembodiments of the present invention.

FIG. 1B is a cross-sectional schematic of portions of a fuel cell inaccordance with embodiments of the present invention.

FIGS. 2A-2D are scanning electron microscopy (SEM) images of fibers spunin accordance with embodiments of the present invention. Images in FIGS.2A-2D are at, respectively, 50, 100, 200, and 500 micron scale.

FIG. 3 shows an SEM image (20 micron scale is shown) of fibers spun inaccordance with embodiments of the present invention after less ripeningtime than the figures shown in FIGS. 2A-2D.

FIG. 4 shows a fiber mat spun with a thickness of about ¼ inch inaccordance with embodiments of the present invention.

FIGS. 5A and 5B compare a silica fiber mat that was electrospun after alonger transitioning time in accordance with embodiments of the presentinvention (FIG. 5A), with a fiber mat electrospun after a shortertransition time in accordance with other embodiments of the presentinvention (FIG. 5B).

FIGS. 6A and 6B show SEM images of fiber dust in accordance withembodiments of the invention, with 100 μm scale shown.

DETAILED DESCRIPTION

In accordance with various embodiments of the present invention, silicafibers and/or powder formed therefrom are utilized as the structuralmatrix (or at least a portion thereof) for various components of a fuelcell. Various components of the fuel cell incorporate other materialsmixed with, applied to, and/or incorporated within the silica fibers inorder to enable the functionality of the fuel cell. The silica fibersthemselves may be produced from a gelatinous material that iselectrospun to form a fiber mat. The mat itself (or a portion thereof)may be utilized within the fuel cell, with or without additionalprocessing (e.g., pressing and/or incorporation of an additivematerial). In various embodiments, the mat is fragmented into a powderor dust, which may include, consist essentially of, or consist offibrous fragments. The powder, which may already incorporate one or moreadditive materials introduced before, during, or after the fiberelectrospinning process, may be utilized in one or more regions of thefuel cell. In various embodiments, the powder is mixed with one or moreadditives for use in one or more fuel-cell regions. In otherembodiments, the powder is pressed into a planar sheet and utilizedwithin the fuel cell, with or without the incorporation of one or moreadditives.

In some embodiments, silica fibers and/or fiber mats are electrospunfrom a gelatinous material. For example, the silica fibers and/or fibermats may be prepared by electrospinning a sol-gel, which may be preparedwith a silicon alkoxide reagent, such as tetraethyl ortho silicate(TEOS), alcohol solvent, and an acid catalyst.

In some embodiments, the sol-gel for preparing the silica fibercomposition is prepared by a method that includes preparing a firstmixture containing an alcohol solvent, a silicon alkoxide reagent suchas tetraethylorthosilicate (TEOS); preparing a second mixture containingan alcohol solvent, water, and an acid catalyst; fully titrating thesecond mixture into the first mixture; and processing (ripening) thecombined mixture to form a gel for electrospinning. In some embodiments,the silicon alkoxide reagent is TEOS. Alternative silicon alkoxidereagents include those with the formula Si(OR)₄, where R is from 1 to 6,and preferably 1, 2, or 3.

In some embodiments, the sol comprises, consists essentially of, orconsists of about 70% to about 90% by weight silicon alkoxide (e.g.,TEOS), about 5% to about 25% by weight alcohol solvent (e.g., anhydrousethanol), an acid catalyst (e.g., less than about 0.1% by weight whenusing HCl) and water. Any sol or sol-gel described herein may includethe balance water (i.e., water may constitute any amount of the sol orsol-gel that is otherwise unspecified). Any sol or sol-gel describedherein may optionally contain one or more reagents or additives that mayor do alter one or more properties of the sol, the sol-gel, and/or thesilica fibers (and/or powder prepared therefrom). Such reagents mayinclude, but are not limited to, for example, polymers and polymericsolutions, inert reagents, alcohols, organic and/or aqueous solvents,organic salts, inorganic salts, metals, metal oxides, metal nitrides,metal oxynitrides, carbon e.g., graphene, graphite, amorphous carbon,fullerenes, etc.), etc.

In some embodiments, the sol contains 70% to 90% tetraethylorthosilicate (TEOS) by weight, 8% to 25% ethanol by weight, 1% to 10%water by weight, and an acid catalyst. In some embodiments, the solcontains 75% to 85% by weight TEOS, 12% to 20% by weight ethanol, andabout 2% to 5% by weight water. An exemplary sol contains about 80% byweight TEOS, about 17% by weight ethanol, and about 3% by weight water.In some embodiments, the acid catalyst is HCl. For example, the sol maycontain less than about 0.1% HCl by weight. For example, the sol maycontain from 0.02% to 0.08% HCl by weight. In various embodiments, thesol does not contain an organic polymer, or other substantial reagents,such that the fiber composition will be substantially pure SiO₂. Invarious embodiments, the sol does not include inorganic salts (e.g.,sodium chloride, lithium chloride, potassium chloride, magnesiumchloride, calcium chloride, and/or barium chloride), nor are, in variousembodiments, inorganic salts mixed with other components of the sol orinto the sol itself. In various embodiments, the fiber composition doesnot include metals or metal oxides (e.g., TiO₂ or ZrO₂). In variousembodiments, the fiber composition consists essentially of SiO₂, i.e.,contains only SiO₂ and unintentional impurities, and, in someembodiments, species and/or complexes resulting from the incompleteconversion of the sol to SiO₂ (e.g., water and/or chemical groups suchas ethoxy groups, silanol groups, hydroxyl groups, etc.). In variousembodiments, additives may be incorporated onto silica fibers and orpowder prepared therefrom after the electrospinning process.

In some embodiments, the alcohol solvent is an anhydrous denaturedethanol, or in some embodiments, methanol, propanol, butanol or anyother suitable alcohol solvent. The first mixture may be agitated, forexample, using a magnetic stirrer, vibration platform or table, or otheragitation means. The second mixture contains an alcohol solvent, water,and an acid catalyst. The alcohol solvent may be an anhydrous denaturedalcohol, or may be methanol, propanol, butanol or any other suitablyprovided alcohol solvent. Water may be distilled water or deionizedwater. Enough acid catalyst is added to the mixture to aid in thereaction. This acid catalyst may be hydrochloric acid, or may besulfuric acid or other suitable acid catalyst. The second mixture may beagitated, for example, magnetic stirrer, vibration platform or table, orother agitation means. In some embodiments, the first mixture (or sol)and the second mixture (or sol) are created without the use of directheat (i.e., heat applied via extrinsic means such as a hot plate orother heat source).

According to various embodiments, the first mixture and the secondmixture are combined by dripping or titrating the second mixture intothe first mixture, preferably with agitation. The combined mixture isthen further processed by allowing the sol to ripen in a controlledenvironment until a substantial portion of the alcohol solvent hasevaporated to create a sol-gel suitable for electrospinning. Forexample, the controlled environment may include an enclosure with atleast one vent and optionally a fan to draw gases away from the mixture,and which may involve controlled conditions in terms of humidity,temperature, and optionally barometric pressure. For example, thehumidity may be controlled (e.g., via use of conventional humidifiersand/or dehumidifiers) within the range of about 30% to about 90%, suchas from about 40% to about 80%, or in some embodiments, from about 50%to about 80%, or from about 50% to about 70% (e.g., about 55%, or about60%, or about 65%). Some humidity may be helpful to slow evaporation ofsolvent, and thereby lengthen the window for successful electrospinning.In some embodiments, the temperature is in the range of from about 50°F. to about 90° F., such as from about 60° F. to about 80° F., or fromabout 65° F. to about 75° F. In various embodiments, the sol is notexposed to heat over 150° F. or heat over 100° F., so as to avoidaccelerating the transition. In some embodiments, barometric pressure isoptionally controlled (e.g., using a low pressure vacuum source such asa pump or a fan). By controlling the environmental conditions duringripening, the time period during which the gel may be electrospun may belengthened; this time period may be a small window of only severalminutes if the ripening process is too accelerated, such as with directheat. When ripening the sol at a constant humidity of about 55% andtemperature of about 72° F., the sol will ripen (gelatinize) in a fewdays, and the window for successful electrospinning may be expanded toat least several hours, and in some embodiments several days. In variousembodiments, the ripening process takes at least 2 days, or at least 3days in some embodiments. However, in various embodiments the ripeningdoes not take more than 10 days, or more than 7 days. In someembodiments, the ripening process takes from 2 to 10 days, or from 2 to7 days, or from 2 to 5 days, or from 2 to 4 days (e.g., about 2, about3, or about 4 days). In various embodiments, the sol-gel is spinnablewell before it transitions into a more solidified, non-flowable mass.

The enclosure space for ripening the sol-gel may include a vent on atleast one surface for exhausting gases from within the enclosure, andoptionally the vent may include a fan for exhausting gases producedduring the ripening process. The enclosure space may optionally includea heating source (e.g., one or more heating elements, for exampleresistive heating elements) for providing a nominal amount of heatwithin the enclosure space, to maintain a preferred temperature. In someembodiments, a source of humidity (e.g., an open container of water orother aqueous, water-based liquid) is provided within the enclosureenvironment to adjust the humidity to a desired range or value. Theenclosure may further include one or more environmental monitors, suchas a temperature reading device (e.g., a thermometer, thermocouple, orother temperature sensor) and/or a humidity reading device (e.g., ahygrometer or other humidity sensor).

In some embodiments, the sol-gel is electrospun after a ripening processof at least 2 days, or at least 36 hours, or at least 3 days, or atleast 4 days, or at least 5 days at the controlled environmentalconditions (but in various embodiments, not more than 10 days or notmore than 7 days under the controlled environmental conditions). Byslowing the ripening process, the ideal time to spin the fibers can beidentified. The weight of the sol-gel may be used as an indicator ofwhen the sol-gel is at or near the ideal time to electrospin. Withoutintending to be bound by theory, it is believed that the viscosity ofthe sol-gel is a poor determinant for identifying the optimal time forelectrospinning. For example, in various embodiments, the sol-gel isfrom about 10% to about 60% of the original weight of the sol (based onloss of alcohol solvent during transitioning). In some embodiments, thesol-gel is from 15 to 50% of the original weight of the sol, or in therange of about 20 to about 40% of the original weight of the sol.

In some embodiments, the sol-gel is ripened for at least 2 days, or atleast 36 hours, or at least 3 days, or at least 4 days, or at least 5days, and is electrospun when the ethylene vapors produced by thecomposition are between about 10% and about 40% of the vapors producedby the starting sol, such as in the range of about 10% and about 25%, orin the range of about 10% to about 20%. Ethylene is a colorlessflammable gas with a faint sweet and musky odor (which is clearlyevident as solvent evaporation slows). Ethylene is produced by thereaction of ethanol and acid. Ethylene may optionally be monitored inthe vapors using a conventional ethylene monitor. In other embodiments,gases produced by the sol during the sol ripening process are monitoredto determine a suitable or optimal time for electrospinning. Gasprofiles may be monitored using gas chromatography.

In various embodiments, additives such as conductive materials and/orcatalyst additives (and/or precursors (e.g., salts) thereof and/orcontaining the additives) may be introduced into the sol-gel prior toelectrospinning, and such additives may therefore be incorporated intoand/or onto the spun fibers. In various embodiments, the additive isintroduced into the sol-gel immediately prior to (e.g., less than 0.5hour before, less than 1 hour before, less than 2 hours before, or lessthan 5 hours before) electrospinning so that the sol-gel successfullyripens prior to introduction of the additive, facilitating successfullyelectrospinning. In various embodiments, the additive may be introducedinto the sol-gel after it has ripened for at least 0.5 days, at least 1day, at least 2 days, or at least 3 days.

In various embodiments, the sol-gel may be ripened for a shorter periodof time, as long as the sol-gel remains spinnable via electrospinning.The resulting silica fiber mat or collection of fibers may in some casesbe more brittle after ripening for a shorter time period, but suchbrittleness may not prevent the fragmenting of the fibers and productionof powder therefrom. In various embodiments, silica fiber powderutilized in one or more regions of the fuel cell may be produced fromsilica fibers or fiber mats electrospun after ripening for less timethan silica fibers or mats utilized within the battery in mat or sheetform. For example, silica fiber powder utilized in one or more regionsof the fuel cell may be produced from silica fibers or fiber matselectrospun after ripening for less than 2 days or less than 1 day.

The processing of the sol-gel mixture may require stirring or otheragitation of the mixtures at various intervals or continuously due tothe development of silicone dioxide crystalline material on the topsurface of the mixtures. This development of crystalline material on thetop surface slows the processing time and it is believed that thecrystalline material seals off exposure of the mixture to the gaseousvacuum provided within the enclosure space. In some embodiments, anysolid crystalline material is removed from the mixture.

Upon completion of the sol-gel process, the sol-gel is then electrospunusing any known technique. The sol or sol-gel may be preserved (e.g.,frozen or refrigerated) if needed (and such time generally will notapply to the time for ripening). An exemplary process forelectrospinning the sol-gel is described in Choi, Sung-Seen, et al.,Silica nanofibers from electrospinning/sol-gel process, Journal ofMaterials Science Letters 22, 2003, 891-893, which is herebyincorporated by reference in its entirety. Exemplary processes forelectrospinning are further disclosed in U.S. Pat. No. 8,088,965, whichis hereby incorporated by reference in its entirety.

In an exemplary electrospinning technique, the sol-gel is placed intoone or more syringe pumps that are fluidly coupled to one or morespinnerets. The spinnerets are connected to a high-voltage (e.g., 5 kVto 50 kV) source and are external to and face toward a groundedcollector drum. The drum rotates during spinning, typically along anaxis of rotation approximately perpendicular to the spinning directionextending from the spinnerets to the drum. As the sol-gel is supplied tothe spinnerets from the syringe pumps (or other holding tank), the highvoltage between the spinnerets and the drum forms charged liquid jetsthat are deposited on the drum as small entangled fibers. As the drumrotates and electrospinning continues, a fibrous mat of silica fibers isformed around the circumference of the drum. In various embodiments, thespinnerets and syringe pump(s) may be disposed on a movable platformthat is movable parallel to the length of the drum. In this manner, thelength along the drum of the resulting fiber mat may be increasedwithout increasing the number of spinnerets. The diameter of the drummay also be increased to increase the areal size of the electrospun mat.The thickness of the mat may be largely dependent upon the amount ofsol-gel used for spinning and thus the amount of electrospinning time.For example, the mat may have a thickness of greater than about ⅛ inch,or greater than about ¼ inch, or greater than about ⅓ inch, or greaterthan about ½ inch.

After completion of the electrospinning process, the resulting mat isremoved from the drum. For example, the mat may be cut and peeled awayfrom the drum in one or more pieces. The mat may then be fragmented toform a powder. In various embodiments, the powder includes, consistsessentially of, or consists of small fibrous fragments that are eachintertwined collections of silica fibers, rather than unitary solidparticles. In some embodiments, the electrospun mat may be fractured,cut, ground, milled, or otherwise divided into small fragments thatmaintain a fibrous structure. In some embodiments, the mat (or one ormore portions thereof) is rubbed through one or more screens or sieves,and the mesh size of the screen determines, at least in part, the sizeof the resulting fibrous fragments or powder or dust produced from theelectrospun mat. For example, the mat or mat portions may be rubbedthrough a succession of two or more screens having decreasing mesh sizes(e.g., screens having mesh numbers of 100, 200, 300, or even 400), inorder to produce a powder or dust or collection of fibrous fragmentshaving the desired sizes.

In various embodiments, one or more additives are introduced onto thesilica fibers during the electrospinning process. For example, a slurrycontaining the material (e.g., in powder or particulate form) may besprayed or misted onto the fibers between the spinnerets and the drum oras formed on the drum itself. In various embodiments, the slurrycontains one or more additives selected for the desired region of thefuel cell in solution with a carrier such as water and/or an organicliquid such as propylene carbonate. Fuel cells in accordance withembodiments of the invention may also incorporate one or more regions orsheets of the spun silica fibers (or powder produced therefrom) withoutthe additive(s).

In various embodiments, the additive may be added into the sol-gel, forexample in particulate or powder form, or as a slurry or mixture, priorto spinning of the silica fibers, and the as-spun fibers willincorporate the additive therein or thereon. In various embodiments, theadditive is added into the sol-gel after at least a portion of theripening time.

In other embodiments, the additive is incorporated onto the silicafibers and/or powder after the fibers or fiber mats are spun. Aftercompletion of the electrospinning process, the resulting mat is removedfrom the drum. For example, the mat may be cut and peeled away from thedrum in one or more pieces. The mat may be cut to size, if desired ornecessary, and the electrospun mat of silica fibers may be coated withone or more additives to form a region of the fuel cell. For example,the additive may be deposited over the silica fibers via techniques suchas electrodeposition from a solution containing the additive, atomiclayer deposition, chemical vapor deposition, or spraying or misting of asolution containing one or more additives selected for the desiredregion of the fuel cell along with a carrier such as water and/or apolymeric binder. In various embodiments, the silica fibers or mat isprocessed into silica fiber powder, and the additive is deposited on thepowder (via, e.g., any of the above techniques) and/or mixed with thepowder.

FIG. 1A is a schematic of a fuel cell in accordance with embodiments ofthe invention. As shown, the fuel cell includes an electrolyte flankedon either side by an anode and a cathode. Fuel (e.g., a hydrogen-basedfuel such as hydrogen gas or a hydrogen-containing gas such as methane)flows along the anode, and air (or, in some embodiments, oxygen gas oranother oxygen-containing gas) flows along the cathode. The fuel isoxidized at the anode, forming (1) protons that migrate through theelectrode and (2) electrons that may be supplied to a load as electricalcurrent. The oxygen is reduced at the cathode to form water that isconducted away from the cathode. Excess heat from the reaction is alsotransported away from the cathode. In various embodiments, the variousregions or portions of the fuel cell are sealed within a housingcontaining the various inlets and outlets, and also one or moreelectrical connections for supplying electrical power to an externalload. As described in more detail below, in various embodiments, theanode of the fuel cell may include, consist essentially of or consist ofa catalyst layer and a diffusion layer, and/or the cathode of the fuelcell may include, consist essentially of, or consist of a catalyst layerand a diffusion layer. In other embodiments, the anode may include,consist essentially of, or consist of a single layer (which maycorrespond to a catalyst layer or a diffusion layer, as describedherein), and/or the cathode may include, consist essentially of, orconsist of a single layer (which may correspond to a catalyst layer or adiffusion layer, as described herein).

FIG. 1B is a more detailed schematic of portions of a fuel cell inaccordance with embodiments of the present invention. As shown, anelectrolyte layer 100 is flanked by an anode-side catalyst layer 110-1and a cathode-side catalyst layer 110-2. Disposed outwardly from thecatalyst layers are an anode-side diffusion layer 120-1 and acathode-side diffusion layer 120-2. In general, the electrolyte layer100 conducts protons therethrough but not electrons in order to preventelectrical shorting of the fuel cell. In addition, the electrolyte layertypically retards or substantially prevents flows of gas therethrough inorder to prevent gas crossover. In various embodiments, the diffusionlayers 120-1, 120-2 are omitted from the structure or (effectively)combined into the catalyst layers 110-1, 110-2, and the resultingstructure may be considered to have an electrolyte layer flanked by ananode layer and a cathode layer, as shown in FIG. 1A.

In various embodiments, the electrolyte layer 100 includes, consistsessentially of, or consists of a mat of silica fibers (e.g., a mat ofsilica fibers produced by electrospinning as detailed herein). Invarious embodiments, the mat of silica fibers incorporates a functionalelectrolyte material to facilitate proton conduction. For example, themat of silica fibers may incorporate liquid phosphoric acid (H₃PO₄)and/or potassium hydroxide to form the electrolyte layer 100. In otherembodiments, the electrolyte layer 100 does not include electrospunsilica fibers; rather, the electrolyte includes, consists essentiallyof, or consists of one or more non-silica (e.g., polymer) membranes orlayers. For example, the membrane may include, consist essentially of,or consist of a polymer or fluoropolymer, e.g., a sulfonatedtetrafluoroethylene based fluoropolymer-copolymer such as NAFION,available from The Chemours Company of Wilmington, Del. In variousembodiments, the membrane may include, consist essentially of, orconsist of polybenzimidazole. During operation, the electrolyte layer100 may incorporate water to facilitate proton conduction. In variousembodiments, the thickness of the electrolyte layer 100 ranges fromapproximately 10 μm to approximately 200 μm. In various embodiments, theelectrolyte layer 100 or a portion thereof (e.g. a non-silica membrane)is porous. For example, pores in the electrolyte layer 100 may range insize from approximately 5 nm to approximately 100 nm.

In various embodiments, electrolyte layers 100 may additionallyincorporate one or more sheets of silica fibers in conjunction with oneor more non-silica membranes. For example, a silica fiber sheet may bedisposed between (and, e.g., in direct mechanical contact with) theelectrolyte layer 100 and the anode-side catalyst layer 110-1, and/or asilica fiber sheet may be disposed between (and, e.g., in directmechanical contact with) the electrolyte layer 100 and the cathode-sidecatalyst layer 110-2. In various embodiments, the electrolyte layer 100includes, consists essentially of, or consists of a silica fiber sheetsandwiched between two non-silica membranes. In various embodiments, theincorporation of one or more sheets of silica fibers provides theelectrolyte layer with protection from thermal decomposition and/ordeformation, enhances mechanical integrity of the structure inside thefuel cell, and/or improves charge mobility within the fuel cell.

In various embodiments, electrolyte layers 100 may include, consistessentially of, or consist of one or more polymeric materials (e.g., oneof the materials described above for non-silica membranes) with silicapowder (e.g., fibrous fragments) incorporated therein and/or thereon.The silica powder itself may incorporate one or more additives orfunctional materials, or such materials may also be mixed within thematrix of the electrolyte layer (e.g., within the polymer material), Inan embodiment, a silica fiber sheet of pressed silica fiber powder(e.g., fibrous fragments) may be sandwiched between two non-silicamembranes, as mentioned above.

In various embodiments, for example those incorporating one or moresilica fiber sheets with the electrolyte layer 100, the fuel cell may besubstantially free of silica fibers and/or silica fiber powder withinone or both catalyst layers 110-1, 110-2 and/or one or both diffusionlayers 120-1, 120-2.

In various embodiments, one or both of the catalyst layers 110-1, 110-2may include, consist essentially of, or consist of a mat of silicafibers incorporating (1) carbon or another electrically conductivematerial and/or (2) a catalyst material such as platinum (e.g., platinumparticles). In various embodiments, the catalyst material (e.g., acatalyst material including, consisting essentially of, or consisting ofone or more metals) may be sufficiently electrically conductive,obviating the need for an additional conductive material. In variousembodiments, one or both of the catalyst layers may include a polymericbinder such as polytetrafluoroethylene (PTFE). In various embodiments,one or both of the catalyst layers may incorporate a sheet of silicafibers and/or silica powder (e.g., fibrous fragments) mixed with and/orincorporating the electrically conductive additive and the catalystadditive and such layers may be mixed with one or more polymericbinders.

In other embodiments, particularly embodiments in which the electrodeincludes silica fibers and/or silica powder, the catalyst layers may notinclude silica fibers; rather, the catalyst layers may include carbon(e.g., carbon paper (i.e., a flat sheet of carbon fibers) or othercarbon-based support material) and a catalyst material such as platinum(e.g., platinum particles) incorporated therein and/or thereon. Invarious embodiments, each of the catalyst layers has a thickness rangingfrom approximately 5 μm to approximately 50 μm, e.g., approximately 10μm to approximately 20 μm. In various embodiments, such catalyst layersmay also include a sheet of silica fibers on one or both sides of thelayer (i.e., between, and in direct contact with, the catalyst layer andthe electrolyte layer, and/or the catalyst layer and the diffusionlayer).

In various embodiments, the diffusion layers 120-1, 120-2 may beomitted, and the catalyst layers 110-1, 110-2 may be considered to be“anode” and “cathode” layers as shown in FIG. 1A. For example, in anembodiment, one or both of the anode layer 110-1 and the cathode layer110-2 may include, consist essentially of, or consist of a sheet ofsilica fibers having a catalyst material (and/or an electricallyconductive material such as carbon) incorporated therein and/or thereon.In an embodiment, the electrolyte layer 100 may include, consistessentially of, or consist of a sheet of silica fibers having noadditive material incorporated therein or thereon. For example, theelectrolyte layer 100 may include, consist essentially of, or consist ofa sheet of silica fibers sandwiched between two non-silica membranes, asdescribed above.

In various embodiments, the diffusion layers 120-1, 120-2, when present,may include, consist essentially of, or consist of a mat of silicafibers incorporating carbon or another electrically conductive material.Desirably, the diffusion layers are electrically conductive and porousto enable gas flow therethrough. In various embodiments, the diffusionlayers may incorporate approximately 10% to approximately 40% PTFE orother fluoropolymer such as NAFION. For example, one or both diffusionlayers may incorporate a polymeric binder material.

In various embodiments, the diffusion layers 120-1, 120-2 may include,consist essentially of, or consist of one or more polymeric materialswith silica powder (e.g., fibrous fragments) incorporated therein and/orthereon. The silica powder itself may incorporate one or more additivesor functional materials, or such materials may also be mixed within thematrix of the diffusion layer (e.g., within the polymer material).

In various embodiments, one or both of the diffusion layers mayincorporate a sheet of silica fibers and/or silica powder (e.g., fibrousfragments) mixed with and/or incorporating the electrically conductiveadditive and such layers may be mixed with one or more polymericbinders.

In other embodiments, particularly embodiments in which the electrodeincludes silica fibers, the diffusion layers may not include silicafibers rather, the catalyst layers may include, consist essentially of,or consist of carbon (e.g., carbon paper or other carbon-based supportmaterial). In various embodiments, each of the diffusion layers has athickness ranging from approximately 10 μm to approximately 100 μm,e.g., approximately 15 μm to approximately 50 μm. In variousembodiments, such diffusion layers may also include a sheet of silicafibers on one or both sides of the layer (i.e., between, and in directcontact with, the catalyst layer and the diffusion layer, and/or on thesurface of the diffusion layer facing away from the catalyst layer).

Once assembled, the fuel cell may be placed in a housing featuringconnections to the anode and cathode in order to provide for poweringexternal loads, as well as inlets and outlets for fuel, air, and/orwater, as shown in FIG. 1A.

EXAMPLES Example 1: Preparation of Silica Fiber Mat

Silica fibers were prepared using an electrospinning process, in which asol-gel was spun onto a collector drum to form a non-woven mat offibers. The sol-gel was made in two parts. First, TEOS was mixed withethanol, and then a second mixture containing HCl, water, and ethanolwas titrated into the mixture. The sol-gel was then allowed to ripen fora few days under controlled conditions before spinning.

In one example, the first sol was made by weighing out 384 grams of TEOS98% and 41.8 grams of anhydrous denatured ethanol, and pouring together.The first sol was allowed to let stand in a beaker, and a magneticstirrer was used to create a homogenous solution. The second sol wasmade by weighing 41.8 grams of anhydrous denatured ethanol, 16.4 gramsof distilled water, and 0.34 grams of hydrochloric acid, which was thenpoured together and mixed for 8 seconds with a magnetic stirrer until ahomogenous second sol was formed.

The second sol was then poured into the titration device, which wasplaced above a beaker containing the first sol. The titration devicethen dripped about 5 drops per second until a third sol was formed viathe mixing of the first sol and the second sol. During the drippingprocess, the first sol was continuously mixed with a magnetic stirrerwhile the second sol was dripped into the first sol.

The combined third sol was then placed into an enclosure box. A lowpressure vacuum was provided by a fan on medium speed to remove fumes.The air temperature within the box was 72° F. with 60% humidity. Thethird sol was allowed to sit and process for about three days. Themixtures were agitated daily to reduce the build-up of crystallinestructures. The third sol began to transition to sol-gel withevaporation of the alcohol solvent. Sol-gel may be monitored todetermine an approximate amount of C₂H₄ (ethylene) in the vapors, whichmay be in the range of about 10-20% relative to that of the original solbefore ripening. Upon proper gelatinization, the sol-gel was loaded intoelectrospinning machine or was frozen to preserve for electrospinning.In this example, proper gelatinization occurred when the total mass ofthe sol-gel was between about 70 grams and about 140 grams. This examplemay be scaled appropriately and the ranges may vary, yet still producedesirable structures. To further identify the ideal time to electropsin,portions of the gel may be dripped into the electric field of thespinning apparatus to evaluate the spinning properties of the sol-gel.

FIGS. 2A-2D are scanning electron microscopy (SEM) images of fibers spunin accordance with embodiments of the invention (50, 100, 200, and 500micron scales shown). As shown, the fibers are flexible, smooth, dense,and continuous (not significantly fractured). FIG. 3 is an SEM image offibers that were electrospun after less ripening time (20 micron scaleshown), where the fibers are clearly rigid with many fractures clearlyevident. Such fibers, in various embodiments, may be more brittle andmore easily processed into silica fiber powder. FIG. 4 shows a fiber matspun in accordance with embodiments of the invention. The flexibilityand continuity of the fibers allows mats to be spun at a thickness of ¼inch or more. The mat has a soft, flexible texture.

FIGS. 5A and 5B are images depicting the variation of properties ofsilica fiber mats as a function of ripening time. The mat of FIG. 5A isillustrative of mats electrospun for at least 2-3 days in accordancewith embodiments of the invention, while the mat of FIG. 5B isillustrative of mats electrospun after less ripening time. The materialin FIG. 5A has a soft texture and is very flexible; such material maystill be processed into fiber dust or used in sheet form. The materialin FIG. 5B is brittle, inflexible, and thin, and may be easily processedinto fiber dust.

A silica fiber mat was fabricated and broken into fragments by rubbingthrough a series of screens of decreasing mesh size. The final screenwas a 200 mesh screen, resulting in fiber dust and/or fibrous fragmentshaving sizes of approximately 20 μm to approximately 200 μm. FIGS. 6Aand 6B show SEM images of the resulting fiber dust, with 100 μm scaleshown.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

The invention claimed is:
 1. A method of fabricating a fuel cell, themethod comprising: providing an electrolyte layer; providing a firstcatalyst layer being configured to receive a hydrogen-containing fuel,oxidize hydrogen from the fuel, and supply protons to the electrolytelayer; providing a second catalyst layer being configured to receiveprotons from the electrolyte layer, receive oxygen, and reduce theoxygen to form water to thereby enable the production of electricalcurrent by the fuel cell; disposing the first catalyst layer on an anodeside of the electrolyte layer; and disposing the second catalyst layeron a cathode side of the electrolyte layer, wherein (i) the firstcatalyst layer comprises a first sheet of silica fibers, the firstcatalyst layer being provided at least in part by (a) producing a firstsol-gel from an initial sol containing 75% to 90%tetraethylorthosilicate (TEOS) by weight, 8% to 25% ethanol by weight,an acid catalyst, and the balance water, and (b) electrospinning thefirst sol-gel, and/or (ii) the second catalyst layer comprises a secondsheet of silica fibers, the second catalyst layer being provided atleast in part by (a) producing a second sol-gel from an initial solcontaining 75% to 90% TEOS by weight, 8% to 25% ethanol by weight, anacid catalyst, and the balance water, and (b) electrospinning the secondsol-gel.
 2. The method of claim 1, wherein at least one of the firstcatalyst layer or the second catalyst layer incorporates a catalystmaterial.
 3. The method of claim 2, wherein the catalyst materialcomprises at least one of platinum, one or more platinum-group metals,nickel, ruthenium, palladium, cerium oxide, or an alloy of platinum withone or more other metals.
 4. The method of claim 1, wherein (i)providing the first catalyst layer comprises, after and/or during theelectrospinning, incorporating a catalyst material onto the first sheetof silica fibers, and/or (ii) providing the second catalyst layercomprises, after and/or during the electrospinning, incorporating acatalyst material onto the second sheet of silica fibers.
 5. The methodof claim 1, further comprising incorporating one or more catalystmaterials, or precursors thereof, into the first sol-gel prior toelectrospinning thereof, wherein the first sheet of silica fiberscomprises the one or more catalyst materials incorporated therein and/orthereon.
 6. The method of claim 1, further comprising incorporating oneor more catalyst materials, or precursors thereof, into the secondsol-gel prior to electrospinning thereof, wherein the second sheet ofsilica fibers comprises the one or more catalyst materials incorporatedtherein and/or thereon.
 7. The method of claim 1, wherein theelectrolyte layer comprises a fluoropolymer, potassium hydroxide, and/orphosphoric acid.
 8. The method of claim 1, wherein the electrolyte layercomprises a non-silica membrane.
 9. The method of claim 1, furthercomprising: disposing a first diffusion layer on the first catalystlayer opposite the electrolyte layer, the first diffusion layer beingconfigured to transport the hydrogen-containing fuel to the firstcatalyst layer; and/or disposing a second diffusion layer on the secondcatalyst layer opposite the electrolyte layer, the second diffusionlayer being configured to transport oxygen to the second catalyst layer.10. The method of claim 9, wherein the first diffusion layer comprises athird sheet of silica fibers, the first diffusion layer being providedat least in part by electrospinning a third sol-gel; and/or wherein thesecond diffusion layer comprises a fourth sheet of silica fibers, thesecond diffusion layer being provided at least in part byelectrospinning a fourth sol-gel.
 11. The method of claim 1, whereinproducing at least one of the first sol-gel or the second sol-gelcomprises ripening the initial sol for at least 2 days at a humidity of40% to 80% and a temperature of 50° F. to 90° F.
 12. The method of claim11, wherein the initial sol is ripened for at least 3 days.
 13. Themethod of claim 11, wherein at least one of the first sol-gel or thesecond sol-gel is electrospun when the weight thereof ranges from 10% to60% of a starting weight of the initial sol prior to ripening.
 14. Themethod of claim 11, wherein at least one of the first sol-gel or thesecond sol-gel is electrospun when production of ethylene vaportherefrom ranges from 10% to 40% relative to the initial sol prior toripening.
 15. A method of fabricating a fuel cell, the methodcomprising: providing an electrolyte layer, the electrolyte layer beingfree of silica fibers; providing a first catalyst layer being configuredto receive a hydrogen-containing fuel, oxidize hydrogen from the fuel,and supply protons to the electrolyte layer; providing a second catalystlayer being configured to receive protons from the electrolyte layer,receive oxygen, and reduce the oxygen to form water to thereby enablethe production of electrical current by the fuel cell; disposing thefirst catalyst layer on and in direct contact with an anode side of theelectrolyte layer; and disposing the second catalyst layer on and indirect contact with a cathode side of the electrolyte layer, wherein (i)the first catalyst layer comprises a first sheet of silica fibers, thefirst catalyst layer being provided at least in part by electrospinninga first sol-gel, and/or (ii) the second catalyst layer comprises asecond sheet of silica fibers, the second catalyst layer being providedat least in part by electrospinning a second sol-gel.
 16. The method ofclaim 15, further comprising, for at least one of the first sol-gel orthe second sol-gel, prior to electrospinning the sol-gel, producing thesol-gel from an initial sol containing 75% to 90%tetraethylorthosilicate (TEOS) by weight, 8% to 25% ethanol by weight,an acid catalyst, and the balance water.
 17. The method of claim 15,wherein the electrolyte layer is porous and non-fibrous.